1. Field of the Invention
The present invention relates to an optical amplifier apparatus, particularly relates to an optical amplifier apparatus that can amplify optically with low pumping power to be high output in a broadband.
2. Description of the Related Art
A wavelength division multiplexing optical transmission system in which signal lights of different plural wavelengths are multiplexed and transmitted is widely used to increase transmission capacity. A batch of optical amplification by an optical amplifier apparatus is also demanded for a signal light (a wavelength multiplexed signal light) acquired by multiplexing such plural signal lights of different wavelengths are multiplexed.
In a conventional type optical amplifier apparatus, an optical equalizer is arranged on the output side of an optical amplifier. The optical equalizer for equalizing the gain of a signal light after optical amplification is finished is arranged. As a result, a problem that the optical output power is reduced by the loss of the optical equalizer and a predetermined output level of a signal light amplified by the optical amplifier apparatus is not acquired is caused.
The optical output power of the optical amplifier is deteriorated by the insertion (the arrangement) of the optical equalizer for a signal light after optical amplification is finished. Therefore, to acquire required optical output power, the power of pumping light from a pumping light source is required to be increased and a demand for the power of pumping light from the pumping light source escalates. The demand has a problem in terms of consumed power, the generation of heat, the life of the pumping light source and others.
3. Summary of the Invention
The object of the invention is to realize an optical amplifier apparatus that can equalize the gain in a broad wavelength band with high precision and can correspond to high output and the reduction of consumed power. Incidentally, the object is to enhance the mounting efficiency of optical components in an optical repeater for which high-density mounting is demanded such as an optical transmitter-receiver, an optical repeater and a submarine optical repeater.
An optical amplifier apparatus according to the invention is provided with a first optical amplifying medium that amplifies an input signal light and outputs a first amplified signal light, a second optical amplifying medium connected to the first optical amplifying medium in series for amplifying the input first amplified signal light and outputting a second amplified signal light and an optical wavelength division multiplexing (WDM) coupler arranged on either side of the side to which a signal light is input of the first optical amplifying medium or the side from which a second optical amplified signal light is output of the second optical amplifying medium for inputting pumping light to the first optical amplifying medium or the second optical amplifying medium. In the configuration described above, an optical equalizer arranged between the first optical amplifying medium and the second optical amplifying medium is provided. The invention is provided with two pumping light sources in the configuration. The two WDM couplers for inputting pumping light respectively output from these pumping light sources to the first and second optical amplifying media are provided. The WDM couplers are composed of a first WDM coupler that is arranged on the side to which a signal light is input of the first optical amplifying medium and inputs first pumping light to the first optical amplifying medium and a second WDM coupler that is arranged on the side from which a second amplified signal light is output of the second optical amplifying medium and inputs second pumping light to the second optical amplifying medium.
The optical equalizer has the dependency upon a wavelength of loss that compensates the dependency upon a wavelength of gain generated when a signal light is amplified by the first and second optical amplifying media. The optical equalizer is characterized in that it is composed by a fiber grating optical equalizer provided with a cutoff wavelength shorter than the wavelength band of pumping light. For example, in case the first and second optical amplifying media are both an erbium-doped fiber, pumping light has a wavelength of 0.98 xcexcm and the optical equalizer can be composed of a fiber grating optical equalizer the cutoff wavelength of which is 0.97 xcexcm or less. The cutoff wavelength of the optical fiber composing a fiber grating is set to 0.97 xcexcm for pumping light having a wavelength of 0.98 xcexcm, however, the cutoff wavelength has only to be a wavelength at which no mode transformation is caused.
The optical amplifier apparatus according to the invention is characterized in that it is provided with a gain control circuit that controls the output of pumping light so that the gain of a second amplified signal light for a signal light is a predetermined value in addition to the configuration described above.
According to the invention, pumping light passes the optical equalizer and pumps the two optical amplifying media by arranging the optical equalizer between the two optical amplifying media. Hereby, as the optical amplifying medium after the optical equalizer amplifies and compensates the loss of the optical equalizer, the optical amplification of high output is acquired.
The mode of pumping light is prevented from being transformed when the pumping light passes the fiber grating optical equalizer by using an optical fiber the cutoff wavelength of which is shorter than the wavelength of pumping light for the optical fiber composing the fiber grating optical equalizer. Concretely, in case an erbium-doped fiber is used for the optical amplifying medium to amplify a signal light having a wavelength of 1.55 xcexcm for example, pumping light having a wavelength of 0.98 xcexcm can be stably propagated without the mode being transformed when the pumping light passes the fiber grating optical equalizer by using pumping light having a wavelength of 0.98 xcexcm and setting the cutoff wavelength of the optical fiber composing the fiber grating optical equalizer to 0.97 xcexcm or less and a stable pumped state is maintained. With the result that the stable pumped state is maintained, the optical amplifier apparatus in which optical output power is stable can be realized.